Similarity, for sure.

But what I consider even more important is COMPATIBILITY. Being similar is compatible but being compatible doesn't always mean being similar.

Compatability is about to understand each other, what they want, understanding their life choices and priorities and respecting them; standing with them in tough times, giving them a shoulder in their tough times.

Two different people can surely survive together happily if they are compatible. Giving my example, me and my best friend are complete opposites of each other. I love to cook, she hates it. She loves to drink and party, that's not my choice. She like adventure, i like shopping. But we are best friends.

Similarly with my sister, we are complete opposites. If she says North, I have to say South without a fail. But she is my best companion. I discuss with her everything, share all my joys and sorrows, like to shop with her and can chit chat with her for hours. That's what I call compatibility.

Respecting others is what is required for a friendship to grow.